1. Field of the Invention
The present invention relates to a system, apparatus and methodology for semiconductor device control, and in particular to controlling semiconductor devices utilizing an isolated FET drive based on Zener diodes.
2. Discussion of the Related Art
Semiconductors have permeated into every aspect of modern society. They are the building blocks used to create everything from the information super-highway to the electronic timer in the family toaster. Generally, any device that is considered “electronic” utilizes semiconductors. These often-unseen components help to reduce the daily workload, increase the safety of our air traffic control systems, and even let us know when it is time to add softener to the washing machine. Modern society has come to rely on these devices in almost every product produced today. And, as we progress further into a technologically dependent society, the demand for increased device speeds, capacity and functionality drive semiconductor manufacturers to push the edge of technology even further.
One common type of semiconductor is the transistor. This device revolutionized the electronics industry after its invention in 1947. Prior to this time, circuits requiring amplification of signals were forced to utilize bulky vacuum tubes for this task. Transistors provided signal amplification at less than a tenth the size of vacuum tubes. This led to new portable devices, such as the transistor radio, that before the transistor could not have been transported easily. Big, bulky pieces of equipment were suddenly reduced in size to handy portable units. Thus, the transistor helped to create a new world of electronic devices that could fit and be utilized in ways never before possible, replacing the fragile and cumbersome vacuum tubes.
Transistor technology has progressed steadily since 1947. Many different types of transistors have been developed such as junction transistors, BJT (bipolar junction transistors), FET (field-effect transistors), MOSFET (metal-oxide semiconductor field-effect transistors) and IGBT (insulated gate bipolar transistors). Generally speaking, a transistor is comprised of semiconductor materials that interface with common physical boundaries. The semiconductor materials utilized include gallium-arsenide and germanium which are doped with impurities to make them conductive. An “n-type” semiconductor has excess electrons due to the impurities and a “p-type” semiconductor has a deficiency of electrons, and therefore, an excess of holes. Electrons are negative charge carriers and holes are positive charge carriers.
A junction transistor consists of two outer semiconductors separated by a thin layer of an opposing type of semiconductor material. When the electric potentials on one of the outer layers and the thin layer meet a certain threshold, a small current between the layers occurs. This small current creates a large current between the two outer semiconductor layers, producing current amplification. Junction transistors can be N-P-N or P-N-P. Either type operates in the same fashion, but each operates with different polarities. The transistor can also be employed as a switch.
Another type of transistor is the BJT that is characterized by two back-to-back p-n junctions which share a common base region. It also has carriers that are injected into the base from the emitter by forward biasing the emitter-base junction (normal operation). The base is much shorter than the carrier diffusion length, and the carriers traverse the base to reach the collector.
The FET was developed after the junction transistor and draws virtually no power from an input signal, surmounting a major obstacle of the junction transistor. A FET is comprised of a channel of semiconductor material interposed between two electrodes. The electrodes attached to the ends of the channel are called the source and the drain. The channel contains regions of opposing semiconductor material to that which makes up the electrodes (p-type versus n-type or n-type versus p-type). These regions are in proximity to electrodes called gates. A specific threshold potential applied to the gates impedes current flow between the source and the drain. This is normally referred to as a reverse potential or voltage, Changing the value of this reverse potential alters the resistance of the channel, allowing the reverse potential to regulate the current flow between the source and the drain. Altering the type of composition of the semiconductor material allows for the device to operate with reversed polarities.
Another variation of the FET transistor is the MOSFET. This is a single gate device in which the gate is separated from the channel by a layer of dielectric, typically metal oxide. The gate's electric field penetrates through the dielectric layer and into the channel, controlling the resistance of the current through the channel. A potential applied to the gate of the MOSFET can increase the current flow between the source and the drain and also decrease it.
Another type of transistor is the IGBT which was developed in the early 1980's to overcome some inherent disadvantages of the MOSFET. The IGBT combines advantages of a BJT and MOSFET to produce advantageous characteristics of both types of transistors. A power MOSFET has simple gate control circuitry capability and fast switching, but rapidly increasing “on” resistance as breakdown voltage is increased. The BJT excels in “on” characteristics, but its base control circuitry needs to be complex for proper operation, and it is not as good in fast switching as the MOSFET. The IGBT combines the best of both types creating a MOS gate input structure employing simple input controls with fast switching and a conduction capability superior to a normal BJT.
Because of the similarities in gate type structures for the MOSFET and IGBT, their gate driver circuitry design can be similar, although biasing needs to be stronger in IGBTs due to the large input gate-to-emitter capacitance. Even though MOSFETs/IGBTs are less complicated to drive than BJTs, care must be taken in the drive circuitry design to maximize the performance of these devices, as well as preventing outright device failure. The MOSFET/IGBT is a voltage controlled device with theoretically no current draw on the gate. In reality, however, there is a large non-linear gate charge that must be overcome before the MOSFET/IGBT is able to turn “ON.” To achieve maximum performance from the MOSFET/IGBT, large instantaneous currents are required to keep switching times and power losses at a minimum. Additionally, most MOSFETs/IGBTs have a typical 20 volt limit on the gate of the MOSFET/IGBT. The silicon dioxide layer between the gate and the source regions can be easily penetrated, resulting in device failure, if the voltage between the gate and source exceeds 20 volts, even with low current. The performance characteristics of MOSFETs/IGBTs depend on well designed gate driver circuitry to function optimally.